The present invention relates to a perpendicular magnetic head, and more particularly, such a magnetic head for recording information signals on a perpendicular magnetic recording medium and for reproducing signals recorded on the recording medium.
Recently, a magnetic head 5 of the auxiliary magnetic pole excitation type as shown in FIG. 1 is considered to have bright future prospects to be used extensively as a perpendicular magnetic head. The magnetic head 5 includes a main magnetic pole 1 formed of a magnetic thin film having high permeability and an auxiliary magnetic pole 4 formed of a magnetic substance block having high permeability on which a winding 3 is provided. Both poles are arranged so as to interpose a magnetic recording medium 2 therebetween. To record information signals, the main magnetic pole 1 is magnetized by a signal current flowing through the winding 3 around the auxiliary magnetic pole 4. As a result, a magnetic layer 2a which extends through the medium of a high permeability layer 2c on a base film 2b of the recording medium 2 is perpendicularly magnetized by the perpendicular magnetic field which develops by the magnetization of the main magnetic pole 1. To reproduce recorded signals, the main magnetic pole 1 is magnetized by the magnetic field which has been developed by the perpendicular magnetization of the recording medium 2 and signal voltages which are induced in the winding 3 by a change in the magnetization of the main magnetic pole 1 are picked up.
The magnetic head 5 features includes a strong interaction between the main magnetic pole 1 and the recording medium 2. Specifically, the magnetic flux from the main magnetic pole 1 is drawn into the recording medium so that only the perpendicular magnetic flux at the tip end of the main magnetic pole 1 is greatly intensified. Since the spread of the magnetic flux is limited by thickness of the main magnetic pole 1, the sharp perpendicular magnetic field is developed regardless of recording levels. In addition, the magnetic head 5 reliably and quite desirabley provides the perpendicular magnetic field to the surface of the magnetic layer 2a of the recording medium 2 practically independently of the positioned accuracy of the center axes position of both the magnetic poles 1, 4.
However, to optimize the recording operation, on one hand, and the reproducing operation, on the other hand, requires different hardware in each case. Since the recording and reproducing operations are performed with the same magnetic head 5, it is difficult to improve their sensitivities. To improve the recording sensitivity, the main magnetic pole 1 should be formed of a soft magnetic film having the high saturation magnetization. To improve the reproducing sensitivity, on the other hand, the main magnetic pole 1 should be formed of a soft magnetic film having a high initial permeability .mu.i.
This can be described with reference to the magnetization curves (hysteresis loops), shown in FIGS. 2A and 2B. FIG. 2A shows the reaction range in the main pole 1 (indicated by the arrow marked "WIDE") to the intensity of the magnetic field during a recording operation mode. FIG. 2B shows the reaction range (indicated by the arror marked "NARROW") to the intensity of the magnetic field induced by the magnetic recording medium 2 during the reproducing operation mode.
As can be seen from FIG. 2A, in the recording mode, relatively wide range of the magnetic field are practically applied by a recording current corresponding to information signals to be recorded, and the magnetic field is applied to the main pole 1 for inducing a concentrated magnetic flux from it so as to perform the perpendicular recording. Therefore, the higher the saturation magnetization (i.e. the saturated magnetic flux density Bs shown in FIG. 2A) of the main pole 1, the higher the strength of the magnetic recording is. On the other hand, in the reproducing process, the intensity of the magnetic field can take merely relatively narrow range as shown in FIG. 2B, since the magnetic field emanates from the magnetic recording medium 2 on which information signal were previously recorded. Therefore the initial permeability .mu.i (i.e. the gradient of the curve in the region of low intensity of the magnetic field shown in FIG. 2B) of the main pole 1 should be high enough for obtaining a high reproducing sensitivity, whereas the saturation magnetization (Bs) can be negligible in the reproducing process. Accordingly, it is best if the main magnetic pole 1 is formed of a substance having both a high saturation magnetization Bs and also having a high initial permeability .mu.i. However, it is difficult, in practice, to provide both simultaneously.
By way of example, an amorphous film made of an alloy of three elements of Co (Cobalt)-Zr (Zirconium)-Nb (Niobium) has a tendency in which when the Co content increases, the saturation magnetization Bs is raised and the initial permeability .mu.i decreases.
In view of the foregoing, conventionally the recording and reproducing magnetic heads are separately provided for carrying out exclusively each function. To this end, it has been proposed to provide the magnetic head shown in FIG. 3. With this head, the recording and reproducing operations use two magnetic heads, an auxiliary pole exciting type recording head 5 and a well known ring type reproducing head 6 with a winding 8 coiled around a ring-shaped magnetic body 7. With the described magnetic head, when the recording density of information is not very high, the ring type head 6 has the good resolving power and the high reproducing sensitivity to read recorded signals satisfactorily. However, when data is recorded at a high density of more than 100 KBPI (10.sup.5 Bit Per Inch), it is difficult to read recorded signals with the ring type head 6. Presently, since a gap width t of the head 6 is approximatel 0.2 .mu.m at the minimum, it is impossible, in principle, to resolve and reproduce a recorded bit of less than that width and the resolving and reproducing sensitivity is only on the level of nearly 0.3 .about.0.4 .mu.m. Therefore, it is necessary for the recording operation of density more than 100 KBPI to employ specifically a reproducing head for exclusive use which is different from the head 6.
Furthermore, there have been proposed other conventional perpendicular magnetic heads disclosed in (1) Japanese Laid-Open Patent Application No. 169,212/1981, (2) Japanese Laid-Open Patent Application No. 203,216/1982 and (3) Japanese Laid-Open Patent Application No. 13,514/1981. The first patent application discloses a magnetic head based fundamentally on using the ring type head 6 and therefore has the disadvantage mentioned above. The second patent application discloses a magnetic head in which a plurality of main magnetic poles are arranged at regular intervals on a rotary member such that video tracks are formed at high speed by rotating the rotary member in a direction perpendicular to direction of travel of the video tape. However, in this magnetic head, although the plurality of main magnetic poles are formed in a unitary body, the recording and reproducing operations are not separated. The last patent application discloses a magnetic head proposed by similar to the present invention in which both recording and reproducing heads are united and a main magnetic pole of the magnetic head is changeable in its thickness between recording and reproducing operations to improve accuracy of the azimuth adjustment and the resaturation resolving-power in the reproducing operation.